Illustrated herein is a method of measuring the lateral charge migration by measuring the average potential of a latent image formed on a photoreceptor surface. It finds particular application in conjunction with detecting lateral charge migration, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable electrophotographic imaging members.
The electrophotographic imaging members may be in the form of plates, drums or flexible belts. These electrophotographic members are usually multi-layered photoreceptors that comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. Materials and methods for producing such photoreceptors are well-known in the art.
The resolution of the final print depends heavily on the location of the electrostatic charge upon the imaging surface of the photoconductive insulating layer. Lateral charge migration (LCM), i.e. the movement of charges on or near the surface of an almost insulating photoconductor surface, has the effect of smoothing out the spatial variations in the surface charge density profile of the latent image. It can be caused by a number of different substances or events (i.e., by ionic contaminants from the environment, by naturally occurring charging device effluents, etc.), which cause the charges to move. LCM can occur locally or over the entire photoconductor surface. As a result, some of the fine features present in the input image may not be present in the final print. This is usually referred to as wipeout or deletion.
Because deletion is undesirable, it is necessary to distinguish acceptable photoconductors (i.e. with no or low LCM) from unacceptable photoconductors (i.e. with high LCM). Often it is not possible, nor desirable, to carry out print tests for deletion; hence, another method is needed. A direct measurement of the latent image profile requires a probe that can detect voltages or fields at the photoconductor surface with a resolution on the scale of tens of microns. Current probes that measure absolute values, i.e., electrostatic voltmeters (ESV), have only a resolution on the order of millimeters. Thus, a resolution improvement of more than an order of magnitude is required for a direct measurement.
Indirect measurements of the LCM are possible. Surface conductivity measurements are commonly used to quantify LCM. However there are problems associated with conventional surface conductivity measurements: 1) they are steady state measurements, 2) the photo-conductor is near insulating and hence, there is the issue of contacts, and 3) in the xerographic process the surface is charged with ions but no ions are usually involved in the traditional steady state surface conductivity measurements.
Furthermore, the only technique that has been used to identify LCM in dual-layer, negatively charged photoreceptors concerned the degree of positive charge acceptance. Photoreceptors with less positive charge acceptance were classified as devices with higher potential for LCM. However, the positive charge acceptance at best only correlates to LCM; there is a degree of error in the correlation. Thus, there is still a need for a better method of quantifying LCM.